Nonlinear optical devices, e.g., frequency doublers, optical mixers and parametric oscillators, are of interest in both research and applied projects because of their ability to convert coherent optical radiation at one frequency into coherent optical radiation at another frequency. This ability is of interest because of the opportunities it affords both for expanding the number of wavelengths at which coherent radiation is available and for converting optical energy to a wavelength more convenient for device applications.
Many materials, such as quartz and lithium niobate, have nonlinear coefficients and exhibit optical nonlinearities. However, because the utility of the material for device applications is generally proportional, at least at relatively low levels of incident radiation, to the magnitude of the nonlinear coefficients, materials which exhibit both nonlinear coefficients larger than presently known nonlinear coefficients and stable operation under diverse conditions, including high intensity incident radiation, are constantly sought.
The possibility of using organic molecules in nonlinear optical devices has generated much interest recently because a large number of molecules are available for investigation. Substituted aromatic molecules have received particular interest because studies, such as Chemical Physics Letters 37 519 (1976), have shown that they may exhibit large optical nonlinearities in the liquid phase. The possibility of such an aromatic molecule having large optical nonlinearities is enhanced if the molecule has donor and acceptor radicals bonded at opposite ends of the conjugated system.
One such substituted aromatic molecule that is potentially interesting from an optical device point of view is para-nitroaniline. This molecule has a large molecular hyperpolarizability, .beta., and is transparent at many wavelengths of interest, including 0.532 .mu.m, which permits frequency doubling of the commonly used 1.064 .mu.m wavelength from a Nd:YAG laser. This molecule, however, crystallizes in a centrosymmetric phase and the second harmonic coefficients are, because of the symmetry conditions, zero.
However, the nonlinear optical coefficients of several noncentrosymmetric molecules, 2-bromo-4-nitroaniline, 2-chloro-4-nitroaniline and (methyl) - (2,4-dinitrophenyl) -amino-2-propanoate, which belong to the Pna2.sub.1, Pna2.sub.1, and P2.sub.1 space groups, respectively, and are closely related to para-nitroaniline, have been investigated. The nonlinear coefficients of the first two molecules in the crystalline phase are discussed in Journal of Applied Physics 43, pp. 2765-2770 (1972) and the nonlinear coefficients of the last molecule are discussed in Journal of Applied Physics 48, pp. 2699-2704 (1977). The largest nonlinear coefficient of the first two molecules is d.sub.223 which has a value of 35 when measured relative to potassium dihydrogen phosphide (KDP) and the largest nonlinear coefficient of the third molecule is d.sub.22 which has a value of approximately 36 when measured relative to the d.sub.11 coefficient of quartz. The ratio of the nonlinear coefficients of KDP and quartz is approximately 1.3.
While characterization of materials by the magnitude of the nonlinear coefficients is useful for many purposes, materials are often more conveniently characterized by a figure of merit, d.sup.2 /n.sup.3, where d is the nonlinear coefficient and n is the index of refraction. The figure of merit contains the material dependent terms characterizing the efficiency of second harmonic generation for relatively low levels of incident radiation. As an example of the magnitudes of typical figures of merit, of the three previously mentioned molecules, crystalline methyl-(2,4-dinitrophenyl)-aminopropanoate has a figure of merit 15 times larger than that of lithium niobate which is generally considered to be a good nonlinear material.